FIG. 11 shows an electric circuit diagram of a prior art resonance type switching power source. This power source comprises a DC power source 1 such as a battery or rectifier circuit of capacitor-input type; a transformer 4 having primary and secondary windings 22, 23; first and second switching elements 2, 3 such as MOS-FETs connected in series to both electrodes of DC power source 1; a series circuit of primary winding 22 of transformer 4 and a current resonance capacitor 15 connected in parallel to first switching element 2 and in series to second switching element 3; a rectifying and smoothing circuit 5 which comprises a rectifying diode 16 and a smoothing capacitor 17 connected to secondary winding 23 of transformer 4; an electric load 10 connected in parallel to smoothing capacitor 17; and a control circuit 20 for producing drive pulses to first and second switching elements 2, 3. The power source also includes first and second parasitic diodes 12, 13 connected in parallel respectively to first and second switching elements 2, 3; and a capacitor 14 connected in parallel to first switching element 2. Control circuit 20 produces outputs to alternately turn on and off first and second switching elements 2, 3 so that electric current flows through primary winding 22 to electrically resonate current resonance capacitor 15 and primary winding 22 when second switching element 3 is turned on while DC power is supplied from secondary winding 23 of transformer 4 through rectifying and smoothing circuit 5 to load 10. When first switching element 2 is tuned off, a closed circuit is formed which involves first switching element 2, primary winding 22 and current resonance capacitor 15. Transformer 4 is a leakage transformer with leakage inductance to form a resonance reactor (not shown) connected in series to primary winding 22.
The power source shown in FIG. 11 also comprises an output voltage detector 11 connected to rectifying and smoothing circuit 5 for detecting output voltages from secondary winding 23, comparing the output voltage with an output reference voltage (not shown), and controlling pulse width of drive pulse signals from control circuit 6 in response to the compared result. Control circuit 20 comprises a drive circuit 21 for supplying drive pulses to each control terminal of first and second switching elements 2, 3; and a PWM circuit 9 for causing drive circuit 21 to produce the drive pulses. Output voltage detector 11 detects the output voltage Vo to load 10 to provide PWM circuit 9 with detection signals so that PWM circuit 9 varies time width of the pulses to drive circuit 21 which thereby alternately turns on and off first and second switching elements 2 and 3 by supplying each control or gate terminal of first and second switching elements 2 and 3 with first and second drive pulse signals in response to the output voltage Vo. As shown in FIG. 11, output voltage detector 11 transports the detection signals to an input terminal of PWM circuit 9 through a photo-coupler of photo-diode 18 and photo-transistor 19.
In operation, control circuit 20 provides each gate terminal of first and second switching elements 2, 3 with first and second drive pulse signals of opposite phase to alternately turn them on and off so that resonance current of substantially sinusoidal waveform flows through primary winding 22 of transformer 4 under resonance action of leakage inductance in transformer 4 and current resonance capacitor 15. Electric current through primary winding 22 produces across secondary winding 23 an inductive voltage which is supplied to load 10 as DC output voltage Vo through rectifying and smoothing circuit 5 of rectifying diode 16 and smoothing capacitor 17. When DC output voltage Vo is higher, a large amount of electric current flows through photo-diode 18, and therefore, photo-transistor 19 receives a larger amount of light from photo-diode 18 so that PWM circuit 9 serves to produce drive pulses of shorter time width to first and second switching elements 2, 3 with the higher DC output voltage Vo. Adversely, when DC output voltage Vo is lower, a small amount of electric current flows through photo-diode 18 so that photo-transistor 19 receives a smaller amount of light from photo-diode 18. Accordingly, PWM circuit 9 serves to produce drive pulses of wider time width to first and second switching elements 2, 3. In this way, PWM circuit 9 adjusts the time width or “on width” of drive pulses to first and second switching elements 2, 3 in response to voltage level of DC output to thereby stabilize DC output voltage Vo to load 10.
In the power source shown in FIG. 11, current on switching rises with the substantially sinusoidal wave form as zero-current switching when first or second switching element 1 or 2 is turned on. Also, voltage on switching rises with the gentle wave form as zero-voltage switching when first or second switching element 1 or 2 is turned off. Zero-current and zero-voltage switching results in reduction of switching loss upon on or off operation of first and second switching element 2, 3. Prior art resonance type switching power sources of similar type are for example shown by Japanese Patent Disclosure No. 11-332232 published Nov. 30, 1999 and Japanese Patent Disclosure No. 2002-171755 published Jun. 14, 2002.
As mentioned-above, prior art resonance type switching power sources utilize series resonance action by reactance component and resonance capacitance of transformer to accomplish zero-current switching, reduction of noise and high efficiency for the power sources. Such control technique is preferably applicable with less change in input voltage, for example, with a narrow range of input voltage such as only 100 volts or only 200 volts or otherwise under PWM control with locked oscillation frequency. However, if input voltage varies in a wide range from 100 to 200 volts, time ratio or duty ratio λ for PWM control steeply changes without change in resonance frequency, but disadvantageously coincidentally producing off-resonance in transformer or interruption of current flow through secondary winding in transformer which results in drop of output voltage or considerable increase of noise.
An object of the present invention is to provide a resonance type switching power source capable of producing a stable output voltage under wider variation of input voltage applied to the power source.